Selective Aerobic Oxidation Of Amines Research Articles

Blue light-driven selective aerobic oxidation of amines by benzothiadiazole metal–organic framework photocatalysis

A zirconium-based benzothiadiazole metal–organic framework, UiO-68-BT, performs blue light-driven selective oxidation of benzyl amines with O2 by photocatalysis.

Tuning the Photocatalytic Activity through Linkages of Porous Conjugated Polymers Based on Triazines for a Selective Oxidation Reaction

Conjugated polymers like conjugated microporous polymers (CMPs) and covalent organic frameworks (COFs) are burgeoning metal‐free photocatalysts for selective oxidation reactions. They possess outstanding features and salient advantages that are inextricably linked to structures at the molecular level. More effort should be devoted to seeking appropriate linkages which can define their crystallinity as well as photocatalytic activity. Herein, with thiazolo[5,4‐d]thiazole (TzTz) and imine as linkages, TzTz‐TA and TTI‐COF are prepared with triazine as the core, respectively. Conclusively, TTI‐COF shows high crystallinity, whereas TzTz‐TA is amorphous, belonging to CMP. However, TzTz‐TA exhibits better photocatalytic activities than TTI‐COF for the blue light‐driven selective aerobic oxidation of amines. The superior photoelectric properties and a more suitable reduction potential are imposed by the TzTz linkages to override the shortcoming of lack of crystallinity for TzTz‐TA. High selectivities for imines by photocatalytic oxidation of benzyl amines are achieved over TzTz‐TA. In addition, TzTz‐TA emerges with remarkable recoverability and recyclable stability. This study implies that tuning the linkages is essential for boosting the photocatalytic activity of porous materials.

Extending the 2D conjugated microporous polymers linked by thiazolo[5,4-d]thiazole for green light-driven selective aerobic oxidation of amines

With pyrene as the donor, a conjugated microporous polymer linked by thiazolo[5,4-d]thiazole (TzTz) is with notable green light-driven performance in the selective aerobic oxidation of benzyl amines.

Hydroxynaphthalene–Nb2O5complex photocatalysis for selective aerobic oxidation of amines induced by blue light

Aerobic oxidation of benzylamine to imine by cooperative photocatalysis of DHNS–Nb2O5and TEMPO is viable.

Surface ligands enhance the catalytic activity of supported Au nanoparticles for the aerobic α-oxidation of amines to amides

Surface ligands control the electronic properties of supported Au nanoparticles and thereby regulate their catalytic activity for the selective aerobic oxidation of amines to amides.

Cooperative Photocatalysis with 4-Amino-TEMPO for Selective Aerobic Oxidation of Amines over TiO2 Nanotubes.

Attaching π-conjugated molecules onto TiO2 can form surface complexes that could capture visible light. However, to make these TiO2 surface complexes durable, integrating 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or its analogues as a redox mediator with photocatalysis is the key to constructing selective chemical transformations. Herein, sodium 6,7-dihydroxynaphthalene-2-sulfonate (DHNS) was obtained by extending the π-conjugated system of catechol by adding a benzene ring and a substituent sodium sulfonate (-SO3 - Na+ ). The DHNS-TiO2 showed the best photocatalytic activity towards the blue light-induced selective aerobic oxidation of benzylamine. Compared to TEMPO, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) could rise above 70% in conversion of benzylamine over the DHNS-TiO2 photocatalyst. Eventually, a wide range of amines could be selectively oxidized into imines with atmospheric O2 by cooperative photocatalysis of DHNS-TiO2 with 4-amino-TEMPO. Notably, superoxide (O2 •- ) is crucial in coupling the photocatalytic cycle of DHNS-TiO2 and the redox cycle of 4-amino-TEMPO. This work underscores the design of surface ligands for semiconductors and the selection of a redox mediator in visible light photocatalysis for selective chemical transformations.

Anchoring dye onto 1D Nb2O5 in cooperation with TEMPO for the selective photocatalytic aerobic oxidation of amines

Visible light-driven photocatalysis over semiconductors has emerged as one of the most desired strategies for animating diverse selective chemical transformations. Herein, one-dimensional (1D) Nb2O5 nanorods with a specific surface area of 264 m2•g−1 were synthesized via a straightforward hydrothermal method. Thereafter, an efficient visible light photocatalyst was constructed by in-situ anchoring of alizarin red S (ARS) onto the Nb2O5 nanorods to afford ARS-Nb2O5. Resultantly, the ARS-Nb2O5 nanorods can facilitate the selective aerobic oxidation of amines into imines, affording about twice of the result over 3D hierarchical Nb2O5. Notably, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) could synchronize the intricate electron and proton transfers between ARS-Nb2O5 nanorods, amines, and superoxide (O2•−). In most cases, the green light-driven primary and secondary amines into imines could be completed with about 95% selectivity within 2 h. This work highlights the dimension of designing semiconductor materials to anchor visible light-active molecules for selective oxidation reactions under benign conditions.

Blue light-powered hydroxynaphthoic acid-titanium dioxide photocatalysis for the selective aerobic oxidation of amines

Solar photocatalysis is the key to resolve many environmental challenges but is usually hard to achieve over a metal oxide semiconductor. Therefore, assembling π-conjugated molecules onto semiconductors becomes an efficient approach to solar conversion via ligand-to-metal charge transfer. Here, a rational design of ligands for titanium dioxide (TiO2) is presented to produce robust visible light photocatalysts. Three hydroxynaphthoic acids (HNAs) were selected as ligands by extending an extra benzene ring of salicylic acid (SA) at 3,4 or 4,5 or 5,6 positions. These ligands could regulate the performance of TiO2 in which 2-hydroxy-1-naphthoic acid (2H1NA) endows the best outcome. In detail, blue light-powered cooperative photocatalysis of 2H1NA-TiO2 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 5 mol%) inaugurates the expeditious formation of imines by oxidation of amines with atmospheric oxygen (O2). Interestingly, the increase of the O2 pressure from 1 atm to 0.4 MPa promoted the selective oxidation of benzylamine but thereafter declined with a further boost to 0.6 MPa. Notably, an electron transfer between the oxidatively quenched 2H1NA-TiO2 and TEMPO is established, offering a new pathway for environmental applications. This work presents a strategy in designing cutting-edge visible light photocatalysts via altering semiconductors with surface ligands.

Cooperative photocatalysis of dye-TiO2 nanotubes with TEMPO+BF4− for selective aerobic oxidation of amines driven by green light

Visible light photocatalysis could offer an eco-friendly alternative for selective transformation of organic molecules. Herein, a cooperative system is created with alizarin red S (ARS), TiO2 nanotubes (TNTs), and an extra redox mediator. The electron transfer between oxidatively quenched ARS-TNTs and the redox mediator is the key to secure cooperative photocatalysis. The selective aerobic oxidation of primary and secondary amines was constructed by cooperative photocatalysis of ARS-TNTs with 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO+BF4−) driven by green light. Specifically, the activity of anatase TNTs could reach about 2.2 times that of the P25 (Aeroxide P25) TiO2 precursor. Due to increased polarity, TEMPO+BF4− serves more efficiently for electron transfer than (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO), conferring above 1.5 times of activity that of TEMPO for the selective aerobic conversion of amines. This work features the potential of designing redox mediators in amplifying cooperative photocatalysis driven by visible light.

Thiazolo[5,4‑d]thiazole linked conjugated microporous polymer photocatalysis for selective aerobic oxidation of amines

Recently, conjugated microporous polymers (CMPs) comprised of thiazolo[5,4-d]thiazole (TzTz) linkages have received much attention due to their excellent photoelectric properties. Herein, the polycondensation of dithiooxamide and benzyl aldehydes of C2, C3, and D2h symmetry afforded three TzTz-linked CMPs, namely TzTz-CMP-1, TzTz-CMP-2, and TzTz-CMP-3. Importantly, the porous and flexible characteristics of TzTz-linked CMPs enable the smooth selective aerobic oxidation of amines in ethanol (C2H5OH), a clean but redox-active solvent. All three TzTz-linked CMPs significantly surpass the benchmark mesoporous graphite carbonnitride (mpg-C3N4) photocatalyst. Intriguingly, TzTz-CMP-2 displays the best photocatalytic activity for the blue-light-mediated selective transformation of primary and secondary amines into imines. The conversions of amines were up to 90% with excellent selectivities for imines. This work highlights that CMPs with TzTz linkages may offer efficient photocatalytic selective transformations under genuinely ambient conditions.

Cooperative TiO2 photocatalysis with TEMPO and N-hydroxysuccinimide for blue light-driven selective aerobic oxidation of amines

TiO2 has been the focus of attention in semiconductor photocatalysis for several decades because it can potentially settle the grand energy and environmental issues with earth-abundant elements of Ti and O. However, because of its wide band gap, TiO2 can only collect UV light, hindering its practical applications under the illumination of sunlight. In view of this, an interesting phenomenon of light-driven adsorption of amines onto TiO2 to form a visible light-absorbing complex was adapted to assemble smart photocatalysis. The endurance of this complex was eminently refurbished by blue light-driven continuous adsorption of amines. This in turn promoted a vital selective chemical transformation, blue light-driven selective oxidation of amines into imines with atmospheric dioxygen (O2). More importantly, the inclusion of TEMPO and N-hydroxysuccinimide (NHS) into the smart photocatalytic system could cooperatively expedite the blue light-driven selective aerobic oxidation of amines into imines through dual independent reaction channels, resembling that of enzymatic catalysis. This work underscores the importance of manoeuvring multiple reaction channels by cooperative photocatalysis during selective chemical transformations.

Molecular design of dye-TiO2 assemblies for green light-induced photocatalytic selective aerobic oxidation of amines

Dye-semiconductor assemblies are very versatile visible light photocatalysts in terms of tunability by tweaking either dye molecules or semiconductor materials. Here, we adopted a strategy of molecular inverse design of alizarin red S (ARS) to identify the blueprint underlying the superior photocatalytic activity of ARS-TiO2 assembly. We discovered that the substituted –OH groups of anthraquinone provide visible light absorption and binding sites. Importantly, the molecular features of 1,2-dihydroxyanthraquinone (1,2-DHA) contributes mostly to the unique photocatalytic activity after binding with TiO2 with broad visible light absorption which can be maintained at high concentration of amines. Moreover, the electron-withdrawing effect of -SO3−Na+ groups increase the acidities of substituted –OH groups, leading to stronger binding and subsequent higher activity. Ultimately, in situ formed 1,2-DHA-TiO2 assembly can be a powerful photocatalyst for green light-induced selective oxidation of amines into imines with aerial O2. This work makes evident the promise of molecular design in tailoring dye-semiconductor assemblies for visible light-induced photocatalytic selective chemical transformations.

Cooperative smart TiO2 photocatalysis and TEMPO catalysis: Visible light-mediated selective aerobic oxidation of amines.

Here, we report a highly efficient cooperative photocatalytic system for the selective oxidation of amines with air. The visible light-assisted adsorption of amines gives rise to a visible light complex on TiO2 that can be self-repaired to safeguard smart photocatalysis. Smart TiO2 photocatalysis works cooperatively with TEMPO catalysis that can perform the visible light-mediated selective oxidation of amines in a swift and recyclable manner. This discovery provides an alternative for addressing environmental challenges by reducing pollutants at the source for oxidative chemical reactions.

Photocatalytic selective aerobic oxidation of amines to nitriles over Ru/γ-Al2O3: the role of the support surface and the strong imine intermediate adsorption

The abundant surface OH groups and electron-deficient Ru sites on Ru/γ-Al2O3 ensure the high selectivity to nitriles under light irradiation.

Polyimide-TiO2 hybrid photocatalysis: Visible light-promoted selective aerobic oxidation of amines

Polyimides are a class of high-performance polymers with good thermal and chemical stability. One specific polyimide can be formed via thermal polymerization of melamine (MA) and pyromellitic dianhydride (PMDA) precursors. Herein, polyimide-TiO2 hybrids were straightforwardly synthesized with a mixture of MA, PMDA and TiO2 by annealing at 325 °C. The crystal phase of anatase TiO2 can be preserved and polyimide can be formed on the surface of TiO2 to enable visible light absorption and high specific surface areas for the polyimide-TiO2 hybrid. Next, this polyimide-TiO2 hybrid as a photocatalyst were successfully applied for visible light-promoted selective aerobic oxidation of amines into imines under irradiation of 460 nm blue light-emitting diodes (LEDs). With 4 mol% of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as the co-catalyst, polyimide-TiO2 hybrid photocatalyst reached 8 times of that of pristine TiO2 for the selective aerobic oxidation of benzylamine. Overall, the cooperative photocatalysis of polyimide-TiO2 hybrid and TEMPO was demonstrably efficient for visible light-promoted selective oxidation of amines with aerial O2 as the oxidant. Moreover, by means of reactive oxygen species (ROS) quenching experiments and in-situ electron paramagnetic resonance (EPR) spectroscopy, superoxide anion radical (O2−) was certified as the crucial ROS in coupling polyimide-TiO2 hybrid photocatalysis and TEMPO catalysis. This work suggests that hybridization of metal oxide semiconductor with polymer like polyimides is viable to forge visible light photocatalysts.

Double-Shell and Flower-Like ZnS–C3N4 Derived from in Situ Supramolecular Self-Assembly for Selective Aerobic Oxidation of Amines to Imines

Multishelled micro-/nanostructures have attracted much research attention because of their unique properties. It is a big challenge to fabricate multishelled hollow structures of complex nanoarchit...

Salicylic acid complexed with TiO2 for visible light-driven selective oxidation of amines into imines with air

The interplay of salicylic acid (SA) and reactive oxygen species (ROS) can modulate biotic and abiotic stress of plants, essential biological aerobic oxidation processes. Meanwhile, ROS plays a critical role in TiO2 photocatalytic system for the degradation of organic species. Herein, we developed a system consisted of SA and TiO2 aiming at the selective oxidation of organic molecules by ROS. Interestingly, SA complexed with TiO2 leads to ligand-to-metal charge transfer (LMCT) under visible light irradiation. The charge transfer from ligand (chemically adsorbed SA) to metal (conduction band of TiO2) activates O2 to ROS, superoxide (O2−). The positive charge located at ligand SA is connected with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) catalysis for direct two-electron oxidation of amines and later regenerated by O2−. SA and its derivatives were screened as ligands of TiO2 for the selective aerobic oxidation of amines into imines under blue light-emitting diode (LED) irradiation in which 5-CH3O-SA (5-methoxysalicylic acid, 0.8 mol%) complexed with anatase TiO2 and coupled with TEMPO (5 mol%) confers significantly better results than the others. By this visible light LMCT route, both primary amines and secondary amines can be selectively oxidized into corresponding imines with atmosphere O2 as the terminal oxidant. Importantly, the desired product of N-benzylidenebenzylamine can be isolated in 92% yield.

Merging the catechol–TiO2 complex photocatalyst with TEMPO for selective aerobic oxidation of amines into imines

Merging of catechol–TiO2 complex photocatalysis with TEMPO catalysis can successfully facilitate the selective oxidation of amines into imines with air under blue LED irradiation.

Visible light photocatalysis of dye-sensitized TiO2: The selective aerobic oxidation of amines to imines

Herein, we report an efficient protocol for the selective oxidation of amines with the sole involvement of conduction band electrons (ecb−) of TiO2 for both the activation of O2 and the ensuing oxidation reaction. Visible light was harvested by dye molecules on the surface of TiO2, which injects electrons into its conduction band, generating dye radical cations. Charge transfers from dye radical cations to amines are relayed by the catalytic cycle of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to TEMPO+, which is an oxidant that can conduct the direct two-electron oxidation of amines to imines. TEMPO was regenerated by the oxidation of TEMPOH with superoxide generated by the interaction of ecb− and O2. This work suggests that one can exploit the strategy of cooperative photocatalysis with each component performing in its optimal role: TiO2 for O2 activation, dye for visible light harvesting, and TEMPO for oxidative transformations.

Simple Copper Catalysts for the Aerobic Oxidation of Amines: Selectivity Control by the Counterion.

We describe the use of simple copper-salt catalysts in the selective aerobic oxidation of amines to nitriles or imines. These catalysts are marked by their exceptional efficiency, operate at ambient temperature and pressure, and allow the oxidation of amines without expensive ligands or additives. This study highlights the significant role counterions can play in controlling selectivity in catalytic aerobic oxidations.